Thermoplastic elastomers have commonly been produced by forming triblock and multiblock copolymers. These types of copolymers can be useful as thermoplastic elastomer ("TPE") compositions due to the presence of "soft" (elastomeric) blocks connecting "hard" (crystallizable or glassy) blocks. The hard blocks bind the polymer network together at typical use temperatures. However, when heated above the melt temperature or glass transition temperature of the hard block, the polymer flows readily exhibiting thermoplastic behavior. See, for example, G. Holden and N. R. Legge, Thermoplastic Elastomers: A Comprehensive Review, Oxford University Press (1987).
The best commercially known class of TPE polymers are the styrenic block copolymers (SBC), typically linear triblock polymers such as styrene-isoprene-styrene and styrene-butadiene-styrene, the latter of which when hydrogenated become essentially styrene-(ethylene-butene)-styrene block copolymers. Radial and star branched SBC copolymers are also well-known. These copolymers typically are prepared by sequential anionic polymerization or by chemical coupling of linear diblock copolymers. The glass transition temperature (T.sub.g) of the typical SBC TPE is equal to or less than about 80-90.degree. C., thus presenting a limitation on the utility of these copolymers under higher temperature use conditions. See, "Structures and Properties of Block Polymers and Multiphase Polymer Systems: An Overview of Present Status and Future Potential", S. L. Aggarwal, Sixth Biennial Manchester Polymer Symposium (UMIST Manchester, March 1976)
Insertion, or coordination, polymerization of olefins can provide economically more efficient means of providing copolymer products, both because of process efficiencies and feedstock cost differences. Thus useful TPE polymers from olefinically unsaturated monomers, such as ethylene and C.sub.3 -C.sub.8 .alpha.-olefins, have been developed and are also well-known. Examples include the physical blends of thermoplastic olefins ("TPO") such as polypropylene with ethylene-propylene copolymers, and similar blends wherein the ethylene-propylene, or ethylene-propylene-diolefin phase is dynamically vulcanized so as to maintain well dispersed, discrete soft phase particles in a polypropylene matrix. See, N. R. Legge, "Thermoplastic elastomer categories: a comparison of physical properties", ELASTOMERICS, pages 14-20 (September, 1991), and references cited therein.
The use of metallocene catalysts for olefin polymerization has led to additional contributions to the field. U.S. Pat. No. 5,391,629 describes thermoplastic elastomer compounds comprising tapered and block linear polymers from ethylene and alpha-olefin monomers. Polymers having hard and soft segments are said to be possible with single site metallocene catalysts that are capable of preparing both segments. Examples are provided of linear thermoplastic elastomers having hard blocks of high density polyethylene or isotactic polypropylene and soft blocks of ethylene-propylene rubber. Japanese Early Publication H4-337308(1992) describes what is said to be a polyolefin copolymer product made by polymerizing propylene first so as to form an isotactic polypropylene and then copolymerizing the polypropylene with ethylene and propylene, both polymerizations in the presence of an organoaluminum compound and a silicon-bridged, biscyclopentadienyl zirconium dihalide compound.
In addition, block-type polymers of polypropylene have been produced which exhibit elastic properties. G. Natta, in an article titled "Properties of Isotactic, Atactic, and Stereoblock Homopolymers, Random and Block Copolymers of .alpha.-Olefins" (Journal of Polymer Science, Vol. 34, pp. 531-549, 1959) reported that an elastomeric polypropylene can be fractionated out of a polymer mixture. The elastomeric properties were attributed to a stereoblock structure comprising alternating isotactic and atactic stereosequences. Similar compositions were disclosed in U.S. Pat. No. 4,335,225. More recently, International Patent WO 95/25757 (Waymouth et al.) described a method for synthesis of elastomeric stereoblock olefin polymers using catalysts which may change their geometry (between a chiral and an achiral geometry) on a time scale that is slower than the rate of monomer insertion, but faster than the average time of a single chain construction. The resulting polymers may have properties ranging from crystalline thermoplastics to thermoplastic elastomers to amorphous gum elastomers depending on ligand type and structure, as well as polymerization conditions.